The invention relates to a method for making a plastic film, the method comprising extruding a plastic film and orientating it.
The invention also relates to an apparatus for making a plastic film, the apparatus comprising an extruder and at least one orientation device for orientating the extruded film.
The invention further relates to a plastic film which comprises bubbles with the maximum diameter of about 100 micrometers and the maximum height of about 10 micrometers.
Making a plastic film by extruding it and then orientating it is known e.g. from U.S. Pat. Nos. 3,244,781 and 3,891,374. It is, however, difficult to make thin and in particular thin foamed films using these solutions.
EP publication 0,182,764 discloses a thin polypropylene film which contains wide and flat disk-like bubbles, which are about 80 micrometers in length and about 50 micrometers in width. The film is produced by extruding material which has been foamed chemically or by means of gas and by orientating the extruded material biaxially. The result is a very versatile plastic film. However, the foaming degree of the film is less than 50%, which is why the properties of the film are not good enough for all purposes.
Furthermore, it is not possible to produce thin films of polymethylpentene or cyclic olefin copolymer using the prior art solutions.
The object of this invention is to provide a very good and thin foamed plastic film and a simple and reliable method and apparatus for making said plastic film.
The method of the invention is characterized in that before extrusion material is mixed into the plastic of the plastic film, cavitation bubbles are formed in the plastic film to be stretched due to the influence of the material, after extrusion the plastic film is orientated by stretching, and simultaneously with orientation pressurized gas is arranged to act on the plastic film so that the gas diffuses in the cavitation bubbles, and thus bubbles containing gas are formed in the plastic film.
The apparatus of the invention is characterized in that the apparatus comprises gas supply means arranged in one orientation device for feeding pressurized gas into the plastic film simultaneously with orientation by stretching so that the fed gas diffuses in the cavitation bubbles that are formed in the plastic film during stretching, and thus bubbles containing gas are formed in the plastic film.
The plastic film of the invention is characterized in that material is mixed into the plastic of the plastic film, cavitation bubbles are formed in the plastic film to be stretched due to the influence of the material, and the plastic film is subjected to stretching and to pressure of pressurized gas simultaneously with stretching so that the bubbles contain said gas, the foaming degree of the plastic film being over 70%.
The basic idea of the invention is that a film is extruded from plastic material by means of an extruder and material has been mixed into the plastic so that when the plastic is stretched, cavitation bubbles are formed in the material particles mixed into the plastic. The film is orientated by stretching and at the same time gas is fed into the film under high pressure so that the gas diffuses in the cavitation bubbles and causes overpressure in them. The idea of a preferred embodiment is that after the first orientation the plastic film is orientated by stretching it in the direction substantially perpendicular to the first orientation direction, and thus the overpressure is released in the cavitation bubbles and the bubbles expand.
An advantage of the invention is that very thin films with a foaming degree of about 70 to 90% can be provided in a relatively simple manner. An advantage of the high foaming degree is that the electric and mechanical properties of the film are very good. A further advantage is that the method and apparatus can be used for making a film for example of polymethylpentene or cyclic olefin copolymer.